Passengers had also expressed concern over the poor English of crew and conditions on other planes leased by Monarch from Lithuanian-owned Small Planet Airlines and Air Italy, and criticised what they considered to be late notification about not travelling on Monarch planes.

"Wet-leasing", by which airlines contract aircraft and flight deck and cabin crew from other companies, is common in the aviation industry, but Monarch's problems could prove damaging in a competitive market. It has been operating more flights out of Birmingham and East Midlands airports after the takeover of BMI by British Airways.

Passengers' outrage at the condition of leased aircraft have been shared on online forums, while one passenger, Paul Weston, contacted the Guardian about the Air Explore plane on which he and his wife flew from Birmingham to Alicante this month. It allegedly had a six-inch piece of gaffer tape below a window, his seat would not go upright and manoeuvring to the gantry at Alicante had been clumsy, said Weston, a "very frequent flyer" on a number of airlines.

He and his wife "reserved seats on an aircraft that looked a very different configuration to the eight or 10 Monarch flights we had previously taken, but we thought nothing of it. We got to the airport and there was no mention of a plane change or a seat change.

"When we arrived at the departure gate there was widespread confusion as the aircraft was from Air Explore and not Monarch. Although unhappy (as were many other passengers), we took the flight and it passed without incident although the aircraft was a 737/400 and very far short of the standard of Monarch's normal A321's. The staff were clearly eastern European although polite and helpful enough."

Weston, from Banbury, Oxfordshire, and a management consultant specialising in customer relations, added the couple later "found an email in our spam folder telling us that the flight had been changed to an alternative carefully selected operator. This was clearly complete rubbish as no one had to change seats and this was always the aircraft they intended to use on the flight."

He accused Monarch of using operators "that are demonstrably not as safe as their own operations", making "no attempt" to highlight this during booking, and "misrepresenting a planned use of an alternative operator as a 'last-minute' change by emailing people a couple of days before their flight".

In a letter of apology to Weston, dated 19 September, Monarch said: "We have also stipulated that the service provided in-flight should be comparable with what you would expect from Monarch. Where this has not been the case, we have investigated the shortfalls experienced and put measures in place to ensure that there is an immediate improvement.

"These include assigning additional Monarch cabin crew representatives to operate on these flights to ensure our usual service standards are delivered. To improve the cabin appearance the aircraft has undergone a through deep clean and daily checks are carried out by our cleaning contractors to ensure standards are maintained." The company offered Weston a £50 voucher.

Monarch told the Guardian the Aurela and Air Explore planes had been deep cleaned after customer feedback. "Although we accept that some of our normal standards have not been met, not all of the comments on third-party websites are accurate … All passengers due to travel on one of the wet-lease aircraft have been notified before travelling and have been given options to change their flight, at no additional cost, if they do not want to travel."